"The Giving Pledge," made public by Bill and Melinda Gates and Warren Buffett four years ago, has already attracted 127 billionaires who have pledged to give at least half of their wealth to philanthropic causes.

CBS News spoke to some of them as well as those behind the initiative. "We don't find a lot of people that say, 'I want to join if it was 40 percent,'" says 83-year-old Buffett in "The Giving Pledge," which was rebroadcast as part of the channel's "60 Minutes" show on Sunday.

Buffett, an American business magnate and investor who is widely considered the most successful investor of the 20th century, is the chairman, CEO and largest shareholder of Berkshire Hathaway. He is consistently ranked among the world's wealthiest people, and has pledged to give away 99 percent of his wealth to philanthropic causes.

Buffett is a friend of President Barack Obama, who called a millionaire tax provision — which could not be implemented — the "Buffett Rule." He has given the bulk of his fortune to the Bill & Melinda Gates Foundation so it can be used to reduce global poverty and disease.

Those who are worth at least a billion dollars and are willing to give half of that away are part of the club. The billionaires who have signed the Giving Pledge are between the ages of 27 and 98.

"We're asking them to step out and to do something big. But a lot of them were already on their way there and just hadn't put a numeric number behind it," adds Melinda, co-founder of the Gates Foundation and the wife of Microsoft founder Bill Gates.

The Gates have committed to giving 95 percent of their wealth away, and have also supported both Democrats and Republicans during election campaigns.

Bill Gates explains the pledge doesn't involve any particular flavor of philanthropy. "There is no pooling of money. We celebrate the diversity of philanthropy."

"My guess is that a very significant percentage of our members, I mean, way over half are going to give a lot more than half," says Buffett.

Jeffrey Skoll is among the first billionaires who responded to the pledge. He was the first employee and also first president of Internet auction firm eBay. He used the wealth to become a philanthropist, particularly through the Skoll Foundation, and found the independent movie production company Participant Media.

Skoll wants to fight certain global threats, including climate change, water security, pandemics, nuclear proliferation and the Middle East conflict.

The pledgers are invited each year to exclusive resorts for sessions on how to give money away more effectively.

Buffett helped convince seven billionaires who signed the pledge to sit down with "60 Minutes." They include investors Pete Peterson and Nicolas Berggruen, South African mining tycoon Patrice Motsepe and his wife, Dr. Precious Moloi-Motsepe, entrepreneur Sara Blakely and AOL founder Steve Case and his wife, Jean.

These pledgers want to help in different areas ranging from unemployment in South Africa to early detection and treatment of brain cancer, and from tax reform in California and the U.S. national debt to helping women.

Philanthropy seems to be growing.

David Callahan, co-founder of the progressive think tank Demos and author of The Cheating Culture, recently started a website, Inside Philanthropy, which provides news on big philanthropy and giving trends, in-depth profiles of major donors in several key sectors, and a new ratings system for foundation program officers.

"I'm continually blown away by the sheer amount of money waiting in the wings and how even major philanthropists like George Soros and Pierre Omidyar have barely scratched the surface in giving away their fortunes," Callahan told Forbes Sunday.

"Bill Gates is a great example," he added. "He's given away more money than anyone, but he keeps getting richer and is now sitting on $79 billion. The philanthropy we've seen in recent years is nothing compared to what's coming."

There are 1,645 billionaires worldwide, with an average net worth of $4.7 billion and with the U.S. the top supplier, according to this year's list of billionaires by Forbes, which also says that Bill Gates, worth about $76 billion, is the richest man in the world.